Single attosecond burst generation during ionization of excited atoms by intense ultrashort laser pulses

We develop an analytical approach to describing the generation of a single attosecond burst during barrier-suppression ionization of a hydrogen atom by an intense laser pulse. We derive analytical expressions that describe the evolution of the electron wave packet in the time interval between the detachment from the atom and the collision with the parent ion for an arbitrary initial atomic state by assuming the atom to be fully ionized in one laser-field half-period. For various s-states, we derive expressions for the profile of the attosecond burst generated when the electron packet collides with the ion and analyze the dependence of its generation efficiency on the principal quantum number n of the initial atomic state. The results obtained are compared with the results of three-dimensional numerical calculations. We show that the attosecond pulse generation efficiency can be several orders of magnitude higher than that in the case of ionization from the ground state when pre-excited atomic states are used.